<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title>demo 2-29</title>
	</head>
	<body>
	</body>
	<script>
		let arr = Array.of(1,2,1,3);
		console.log(arr.findIndex(item => item == 2));
		console.log([,1].findIndex(n => true));
	</script>
	
</html>